The present invention relates to a device having two terminals interconnected by one or more material layers for switching between a current conducting state and a state of blocking transport of charge carriers between the terminals upon applying a voltage thereacross.
Accordingly, the invention relates to a switching device in the broadest sense of this expression and is directed to all types of devices adapted to assume a state, in which they are conducting when a voltage is applied across the terminals and a state in which a transport of charge carriers between the terminals is blocked in spite of a voltage being applied across the terminals. The device may be of the type, in which the voltage in the conducting state has an opposite direction than in the blocking state. The simplest device of this type is a rectifying diode, but the device may also be of the type capable to assume either a conducting state or a blocking state when a voltage is applied across the terminals in one and the same direction.
The definition "terminals interconnected by one or more material layers" is used for limiting the invention with respect to circuit breakers or switching devices obtaining the switching action by breaking and establishing a physical connection between the terminals of the device, i.e. connecting and disconnecting them. In contrast thereto, the physical properties of the material layers in connection with the availability of charge carriers and voltage will decide the state of the device.
A device of this type finds many applications, but the use of such a device for high power applications will hereinafter be discussed for clarity, but not in any way restricting the invention.
A device of this type may be used in equipment for handling high electric power for switching high voltages and currents for instance in circuit breakers, commutators, current valves, surge diverters, current limiters and the like. The breakdown voltage of such a device is in most of these applications considerably lower than the voltage to be held by the position in which the device is arranged in the equipment, so that it is necessary to connect a comparatively large number of such devices in series for distributing the total voltage them. The total voltage may well exceed 100 kV, whereas a single device may for instance have a breakdown voltage of 2-5 kV. A complicated and thus costly equipment is required for controlling such devices. Also, equipment for cooling them has to be rather sophisticated and expensive, especially in high frequency operation, for instance when the device is used in current valves switched according to Pulse Width Modulation (PWM) in converter stations. In fact, the major part of the costs for a converter station is caused by the controlling and cooling equipment, so that it is highly desired to reduce the number of devices required in such stations and other high power applications for saving costs.